Implantable stimulation devices deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability with any implantable medical device or in any implantable medical device system.
A SCS system typically includes an Implantable Pulse Generator (IPG) which has a biocompatible device case formed of a conductive material such as titanium, for example. The case typically holds the circuitry of the IPG and a battery to provide power to the circuitry. Depending on the particular needs and circumstances of the patient who will be using the IPG, the battery can be either rechargeable or a non-rechargeable primary battery.
Although many IPGs use rechargeable batteries, there are situations in which use of a primary battery may be advantageous. A primary battery is one in which the electrochemical reaction is not reversible by passing a charging current therethrough, thus rendering the battery non-rechargeable. Primary batteries use up the materials in one or both of their electrodes and thus have a limited life span, but they are typically cheaper than rechargeable batteries, and may not suffer from the same reliability concerns. As such, the use of primary batteries in a medical implantable device is preferred when appropriate, for example, when the expected life of the primary battery would be expected to exceed the patient's life expectancy, or in situations where patients with physical or mental limitations would have difficulty charging the battery. Use of a primary battery in an IPG, however, creates a challenge in the design and construction of the IPG, as a primary battery is generally larger in size than a rechargeable one, and it is not optimal to increase the size of the IPG.